Cathode-ray tube having an auxiliary electrode between the control grid and the anode electrode



Nov. 29, 1966 c. WEBER ET AL 3,289,034

CATHODERAY TUBE HAVING AN AUXILIARY ELECTRODE BETWEEN THE CONTROL GRID AND THE ANODE ELECTRODE Original Filed April 27, 1962 INVENTOR CORNELIS WEBER JOHANNES VAN ESDONK AGENT United States Patent 3,289,034 CATHODE-RAY TUBE HAVING AN AUXILIARY ELECTRODE BETWEEN THE CONTROL GRID AND THE ANODE ELECTRODE Cornelis Weber and Johannes van Esdonk, both of Emmasingel, Eindhoven, Netherlands, assiguors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Continuation of application Ser. No. 190,582, Apr. 27, 1962. This application May 4, 1966, Ser. No. 548,819 Claims priority, application Netherlands, June 24, 1961,

,121 4 Claims. (Cl. 315-31) The application is a continuation of application Serial No. 190,582, filed April 27, 1962, now abandoned.

This invention relates to devices comprising a cathoderay tube having an electron gun which can be operated at low control-voltages, it being possible to obtain an electron beam producing a spot of very small diameter on the fluorescent screen. The invention also relates to cathode-ray tubes for such devices.

In order to obtain a high control sensitivity, it is known that a fine-meshed grid electrode maintained at a low positive potential may be arranged closely in front of the cathode. The positive potential is necessary to allow the passage of a suflicient flow of electrons despite the fine meshes. In certain cases an auxiliary electrode maintained at zero potential of a low negative potential is arranged between the said grid and the anode maintained at a high potential, so that the electron current is completely suppressed if the grid has a potential equal to that of the cathode. The cross-section of the ray near the control electrode may then be determined by either the cathode surface or the aperture of the grid.

However, a positive grid has a limitation in that it starts to convey current. But if the positive potential is maintained low and the grid is made from very thin wires and arranged close to the cathode, both the number and the velocity of the electrons striking the grid wires may be limited, so that their energy is low and the heating of the grid remains within permissible limits. The grid is usually wound on a thin plate secured to a rigid frame.

The positive grid potential involves the further disadvantage that straying of the electrons at right angles to the grid wires is apt to occur so that the electron spot on the fluorescent screen of the tube is elliptically deformed as a function of the instantaneous value of the control voltages.

It also is known to reduce the elliptic deformation of the spot by replacing the grid with a thin perforated plate, but this results in greatly increased dimensions of the luminous spot, which is likewise undesirable.

In accordance With the invention, the said disadvantages are greatly reduced and a high control sensitivity obtained while producing a small luminous spot on the screen if in a device according to the invention comprising a cathode-ray tube having an electron gun constituted by a cathode, a grid electrode maintained at a low positive potential is arranged closely in front of the cathode, an auxiliary electrode maintained at a negative potential, and an anode maintained at a high positive potential, in which steps have been taken to ensure that the cathode-ray, upon leaving the grid, has a predetermined crosssection and produces a spot of very small size on a fluorescent screen, so high a negative potential is applied to the auxiliary electrode that the space potential at the grid surface is substantially equal to the positive potential of the grid, and the aperture of the auxiliary electrode has a diameter at least about thrice that of the active grid surface. The negative potential of the auxiliary electrode is higher than 50 volts and usually from Patented Nov. 29, 1936 "ice 100 to 150 volts negative with respect to the mean cathode potential, dependent upon the anode voltage and the dimensions and arrangement of the electrodes of the gun.

The cross-section of the cathode ray upon leaving the grid may be determined by the electron-emissive cathode surface, or by the aperture in the grid electrode itself.

The invention will now be described with reference to the accompanying drawing in which:

FIGURE 1 shows a device according to the invention;

FIGURE 2 shows a detail of this device, and

FIGURE 3 shows a detail of another embodiment of the invention.

In FIGURE 1 the reference numeral 1 indicates the envelope of a cathode-ray tube. The tube contains an electron gun comprising a cathode 2, a grid 3, an anode 4, an auxiliary electrode 5 and a screen electrode 6, and further a fluorescent screen '7.

The control voltage is applied in this case between ground and the cathode 2. The cathode 2 has a surface area which corresponds to the cross-section of the beam and thus determines the cross-sections near the grid. Interference resulting from electrons released from the margin of cathode 2 are avoided since emission of electrons from the margin is suppressed by applying a negative potential which is not critical, and may be from 2 to 20 volts negative to screen electrode 6 tightly surrounding the electron-emissive surface. That surface of the screen electrode which is adjacent the grid may structurally in the best way be co-planar with the active surface of the cathode.

Since grid 3 has a slightly positive potential, for example of 10 volts, and steps have been taken that the space potential in situ is substantially equal to the grid potential, it is as if the grid wires were not present and the electrons are substantially not deflected by the grid wires. Due to the small distance between grid 3 and cathode 2 of, for example, from to microns and the low positive potential of grid 3, the electrons striking the grid wires have a low velocity and hence a low energy (about 1 mw. in this case) so that disadvantageous heating of the thin grid wires does not occur. Since the grid wires substantially do not produce a lens effect and the space potential is substantially equal to the grid potential, the electrons are focussed at a single focus between auxiliary electrode 5 and anode 4, as shown in FIGURE 2.

Since the electrons in passing through the grid have a low velocity, the control is sensitive. In order to prevent an excessive increase in the diameter of the beam as a result of mutual repulsion of the electrons by the space charge in the beam, it is desirable for the electrons, after having passed the grid, rapidly to attain a high velocity. It is therefore desirable that the anode 4, having applied to it a high potential, should not be arranged too far from grid 3. If the anode 4 were arranged at a large distance in order to obtain the low space potential near the grid, the electrons would travel over a large distance at a comparatively low speed, resulting in straying of the cathoderay. Despite the comparatively small distance between anode 4 and grid 2 it has been found possible for the space potential at the grid still to be made sufficiently low by applying a high negative potential to auxiliary electrode 5.

In order to ensure, in addition, that the equipotential lines near the grid are substantially parallel to the active grid surface, as is necessary for preventing deflection of the marginal electrons of the beam, the aperture of the auxiliary electrode 5 must have a diameter at least thrice the diameter of the ray at the grid 3.

A difficulty involved in the structure of FIGURE 2, in which the cross-section of the cathode ray is determined by the cathode surface, is that the cathode must be positioned very accurately in the center line of the gun.

This disadvantage is avoided if the cathode has, in a manner known per se, a surface area, larger than the cross-section of the ray near the grid, which cross-section is determined by the open portion of the grid, as shown in FIGURE 3. The cathode 8 need now be positioned only at the correct distance from grid 9, but in a lateral direction the mounting is not critical.

The grid 9 comprises ten thin wires provided over the aperture of a thin metal plate 10. The plate 10 must be thinner than one tenth of the diameter of the ray in situ, since the edge of the aperture in the plate otherwise causes interference in the electron paths. The aperture of grid 9 is in this case 0.4 mm. in diameter, the grid wires 7 are 7 microns thick, the plate 10 is 10 microns thick and the pitch of the grid Wires is 40 microns.

The plate 10 itself is secured to a rigid grid frame 11 the aperture of which is such that the distance between its edge and the cathode ray is at least twice the thickness of the frame. In this case the aperture of frame 11 has a diameter of 2 mm. This distance between grid 9 and cathode surface 8 is 90 microns, the distance between auxiliary electrode and cathode 8 is 650 microns. The cylindrical part of auxiliary electrode 5 has a diameter of mm. and the smallest distance between anode 4 and cathode 8 is about 5 mm.

As shown in FIGURES 2 and 3, the cathode ray has a single substantially circular punctiforxn focus 12, in contrast to the case where the grid is not maintained at the space potential locally prevailing and the focus is deformed elliptically. A focus of unduly large size occurs if the apertures of the electrode parts 5, 10 and 11 are not large enough, as is the case in most conventional electron guns. Too large an aperture of electrode 5 is disadvantageous, however, since in this case its potential must be much more negative for obtaining the desired low space potential near the grids 3 and 9 respectively.

It has been found that a device according to the invention permits of substantially avoiding the disadvantageous deformation of the luminous spot on screen 7, while a control voltage less than 10 volts suffices for complete modulation of the beam current.

What is claimed is:

1. An image-reproducing device comprising a cathoderay tube having a beam-forming electrode system comprising a cathode, a first anode, means to apply a positive potential to said anode, a disc-shaped control electrode having an aperture for the passage of a cathode-ray positioned in proximity to the cathode between the anode and cathode, a plurality of parallel wire conductors extending across said aperture in said control electrode, means to apply to said control electrode a positive potential less than that applied to said anode, an auxiliary electrode having an aperture whose diameter is at least three times that of the cathode-ray at the control electrode positioned between said control electrode and said anode, and means to apply a negative potential of more than 50 volts to said auxiliary electrode.

2. A cathode-ray tube as claimed in claim 1, in which a screen electrode surrounds the cathode in close proximity thereto.

3, An image-reproducing device comprising a cathoderay tube having a beam-forming electrode system comprising a cathode having an electron-emissive surface larger than the cross-section of a cathode-ray, a first anode, means to apply a positive potential to said anode, a discshaped control electrode having an aperture for the passage of a cathode-ray positioned between the anode and cathode and in proximity to the cathode, said disc having a thickness which is less than one-tenth of the diameter of the cathode-ray, the cross-section of which is determined by the aperture in the disc, 21 frame supporting said disc having an aperture the edge of which is spaced from the cathode-ray a distance which is at least twice the thickness of the frame, a plurality of parallel wire conductors extending across the aperture in said disc, means to apply to the control electrode a positive potential lower than that applied to the anode, an auxiliary electrode having an aperture whose diameter is at least three times that of the cathode-ray at the control electrode positioned between said control electrode and said anode, and means to apply to said auxiliary electrode a negative potential of more than 50 volts.

4. A cathode-ray tube as claimed in claim 3, in which the control electrode is spaced from the cathode emissive surface a distance of about the control electrode being spaced from the auxiliary electrode a distance of about 650/L, the spacing between the cathode and anode being about 5 mm., the aperture in the control electrode disc being about 400 in diameter, and the aperture of the frame supporting the control electrode and that of the auxiliary electrode being about 2 mm, in diameter.

References Cited by the Examiner UNITED STATES PATENTS 2,225,455 12/1940 Klauer 3l38l X 2,852,716 9/ 1958 Lafferty 3l382.1 X 2,922,072 1/1960 Collins et al. 3 l3-82 X 2,975,315 3/ 1961 Sz/eglio 313-82 2,983,842 5/1961 Hrbek 31382 HERMAN KARL SAALBACH, Primary Examiner.

GEORGE N. WESTBY, S. CHATMON, JR., 4 Assistant Examiners. 

1. AN IMAGE-REPRODUCING DEVICE COMPRISING A CATHODERAY TUBE HAVING A BEAM-FORMING ELECTRODE SYSTEM COMPRISING A CATHODE, A FIRST ANODE, MEANS TO APPLY A POSITIVE POTENTIAL TO SAID ANODE, A DISC-SHAPED CONTROL ELECTRODE HAVING AN APERTURE FOR THE PASSAGE OF THE CATHODE-RAY POSITIONED IN PROXIMITY TO THE CATHODE BETWEEN THE ANODE AND CATHODE, A PLURALITY OF PARALLEL WIRE CONDUCTORS EXTENDING ACROSS SAID APERTURE IN SAID CONTROL ELECTRODE, MEANS TO APPLY TO SAID CONTROL ELECTRODE A POSITIVE POTENTIAL LESS THAN THAT APPLIED TO SAID ANODE, AN AUXILIARY ELECTRODE HAVING AN APERTURE WHOSE DIAMETER IS AT LEAST THREE TIMES THAT OF THE CATHODE-RAY AT THE CONTROL ELECTRODE POSITIONED BETWEEN SAID CONTAOL ELECTRODE AND SAID ANODE, AND MEANS TO APPLY A NEGATIVE POTENTIAL OF MORE THAN -50 VOLTS TO SAID AUXILIARY ELECTRODE. 